Control apparatus and control method for internal combusion engine

ABSTRACT

A control apparatus for an internal combustion engine includes a controller that performs a variable valve timing control to change a valve timing of an intake valve. A condition for performing the variable valve timing control when the engine is cold includes a condition that a control of increasing a fuel injection amount is being performed.

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2011-112509 filed on May 19, 2011 including the specification, drawings and abstract is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a control apparatus and a control method for an internal combustion engine that perform a variable valve timing control to change a valve timing of an intake valve.

2. Description of Related Art

A variable valve timing mechanism that changes the valve timing of an intake valve has been put into practical use as a mechanism that is mounted on an internal combustion engine installed in a vehicle or the like. In the internal combustion engine that is provided with this variable valve timing mechanism, a variable valve timing control that changes the valve timing of the intake valve in accordance with the engine operating state is performed.

Conventionally, in Japanese Patent Application Publication No. 2010-223138 (JP-2010-223138 A), there is described a control apparatus for an internal combustion engine, which permits the variable valve timing control of the intake valve when the engine is cold on the condition that the return of the valve timing to its initial position is guaranteed.

If the valve overlap of the intake valve and an exhaust valve is caused through the advancement of the valve timing by performing the variable valve timing control when the engine is cold as well, an intake port is warmed through the blowback of combustion gas to the intake port, and the atomization of fuel can be promoted. Therefore, combustion is improved and exhaust emission properties are improved. However, the following problems about driveability may arise if the variable valve timing control is performed when the engine is cold.

When the engine is cold, namely, during a period from a time when the engine is started to a time when the warm-up of the engine is completed, a wall surface of the port is cold. Therefore, the amount of fuel that has been injected from an injector and adhered to the wall surface of the port increases. When the variable valve timing control is performed in this state to advance the valve timing of the intake valve and increase the valve overlap of the intake valve and the exhaust valve, combustion gas is blown back from a combustion chamber to the intake port. Then, the fuel that has adhered to the wall surface of the port is gasified by the combustion gas. Therefore, the air-fuel ratio of the mixture burned in the combustion chamber is enriched, and idling stability may deteriorate.

Further, in some cases where the variable valve timing control is performed when the engine is cold, the air-fuel ratio during acceleration becomes lean to cause deterioration in acceleration ability (hesitation). When an accelerator pedal is depressed to increase the amount of intake air, the fuel that has adhered to the wall surface of the port is gasified by airflow, and is introduced into the combustion chamber. The acceleration ability of the vehicle is ensured taking into account an increase in the amount of burned fuel resulting from such gasification from the wall surface of the port. On the other hand, when the valve timing of the intake valve is advanced before acceleration, the fuel that has adhered to the wall surface of the port is gasified due to the blowback of combustion gas. Therefore, the amount of the fuel that has adhered to the wall surface of the port decreases. Thus, when the valve timing of the intake valve is advanced before acceleration, the amount of burned fuel during acceleration is smaller than expected, and torque cannot be sufficiently increased.

Thus, if the variable valve timing control of the intake valve is permitted simply because the return of the valve timing to its initial position is guaranteed when the engine is cold, the air-fuel ratio becomes rich due to the advancement of the valve timing, or the air-fuel ratio becomes lean during acceleration, so that driveability may deteriorate.

SUMMARY OF THE INVENTION

The invention provides a control apparatus and a control method for an internal combustion engine that can perform a variable valve timing control when the engine is cold without increasing the range of deterioration in driveability.

A first aspect of the invention relates to a control apparatus for an internal combustion engine. This control apparatus includes a controller that performs a variable valve timing control to change a valve timing of an intake valve. In this control apparatus, a condition for performing the variable valve timing control when the engine is cold includes a condition that a control of increasing a fuel injection amount is being performed.

If the variable valve timing control is unconditionally performed when the engine is cold, the air-fuel ratio becomes rich due to the advancement of the valve timing, or the air-fuel ratio becomes lean during acceleration as described above, so that driveability may be deteriorated. In order to avoid this phenomenon, it is desirable to prohibit the variable valve timing control when the engine is cold.

On the other hand, when the control of increasing the fuel injection amount is being performed, the air-fuel ratio has been enriched through the control of increasing the fuel injection amount, and deterioration in driveability has already been caused. Thus, when the control of increasing the fuel injection amount is being performed, the variable valve timing control can be performed without the need of worrying that deterioration in driveability is caused. Further, since the fuel injection amount has been increased, the amount of burned fuel does not become insufficient during acceleration. Thus, according to the foregoing configuration, the variable valve timing control can be performed when the engine is cold, without increasing the range of the deterioration in driveability.

The condition for performing the variable valve timing control when the engine is cold includes the condition that the control of increasing the fuel injection amount is being performed, and examples of the control of increasing the fuel injection amount include the control of increasing the fuel injection amount to raise the temperature of a catalyst. The condition for performing the variable valve timing control when the engine is cold may include the condition that the control of increasing the fuel injection amount for a purpose other than the purpose of raising the temperature of the catalyst is being performed.

It should be noted that when the control of increasing the fuel injection amount and the variable valve timing control are started at the same time, the enrichment of the air-fuel ratio resulting from an increase in the fuel injection amount and the enrichment of the air-fuel ratio resulting from the advancement of the valve timing start simultaneously, and an abrupt increase in torque may be caused. Thus, the condition for performing the variable valve timing control when the engine is cold may include a condition that a prescribed time has elapsed since the beginning of the control of increasing the fuel injection amount. In this configuration, a time lag occurs between the beginning of the control of increasing the fuel injection amount and the beginning of the variable valve timing control. Thus, an abrupt increase in torque can be suppressed.

On the other hand, if the internal combustion engine is repeatedly started before the lapse of an engine stop time that is sufficient to evaporate the fuel that has adhered to the intake port, the amount of the fuel that has adhered to the intake port increases every time the internal combustion engine is started. Then, if the variable valve timing control is performed with an excessive amount of fuel adhering to the intake port when the engine is cold, the air-fuel ratio is greatly enriched, and a combustion failure may be caused. Thus, the condition for performing the variable valve timing control when the engine is cold may include a condition that the number of times of repeated start of the internal combustion engine is equal to or smaller than a prescribed value. In this configuration, if an excessive amount of fuel adheres to the intake port, the variable valve timing control is not permitted. Thus, the air-fuel ratio can be prevented from being greatly enriched through the performance of the variable valve timing control when the engine is cold.

A second aspect of the invention relates to a control method for an internal combustion engine. This control method includes determining whether or not a condition for performing a variable valve timing control when the engine is cold is fulfilled, and performing the variable valve timing control if it is determined that the condition for performing the variable valve timing control when the engine is cold is fulfilled. The condition for performing the variable valve timing control when the engine is cold includes a condition that a control of increasing a fuel injection amount is being performed.

BRIEF DESCRIPTION OF THE DRAWINGS

Features, advantages, and technical and industrial significance of an exemplary embodiment of the invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein:

FIG. 1 is a schematic view schematically showing the configuration of an internal combustion engine to which the embodiment of the invention is applied;

FIG. 2 is a flowchart showing a processing procedure of a cold VVT control precondition fulfillment determination routine that is adopted in the embodiment of the invention;

FIG. 3 is a time chart showing an example of a control mode during the start of the engine in the embodiment of the invention;

FIG. 4 is a time chart showing an example of a control mode before and after the performance of an AI control in the embodiment of the invention; and

FIG. 5 is a time chart showing an example of a control mode in a plurality of trips in the embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENT

One embodiment of the invention as a concrete form of a control apparatus for an internal combustion engine according to the invention will be described hereinafter in detail with reference to FIGS. 1 to 5. As shown in FIG. 1, in an intake passage 1 of an internal combustion engine to which a control apparatus according to this embodiment of the invention is applied, an air cleaner 2 that purifies intake air, an airflow meter 3 that detects an amount of intake air, a throttle valve 4 that adjusts an amount of intake air, and an injector 5 that injects fuel into intake air are sequentially disposed from an upstream region of the intake passage 1. Besides, the intake passage 1 is connected to a combustion chamber 8 via an intake valve 6. It should be noted that this internal combustion engine is provided with a variable valve timing mechanism (a VVT mechanism) 7 that changes a valve timing of the intake valve 6.

An ignition plug 9 that ignites a mixture of intake air and fuel is installed in the combustion chamber 8 in which the mixture is burned. Besides, the combustion chamber 8 is connected to an exhaust passage 11 via an exhaust valve 10.

In the exhaust passage 11 through which exhaust gas discharged from the combustion chamber 8 flows, an air-fuel ratio sensor 12 that detects a concentration of oxygen in exhaust gas, and a catalyst 13 that purifies exhaust gas are disposed. Further, a secondary air supply channel 14 for supplying secondary air into exhaust gas is connected to the exhaust passage 11. In the secondary air supply channel 14, an air pump 15 that pressurizes and discharges air, and an air switching valve 16 that allows and interrupts the supply of secondary air are disposed.

This internal combustion engine is controlled by an electronic control unit 17. The electronic control unit 17 includes a central processing unit (a CPU) that performs various kinds of calculation processing related to engine control, a read-only memory (a ROM) in which programs and data for engine control are stored, and a random access memory (a RAM) in which a calculation result of the CPU, detection results of sensors, and the like are temporarily stored.

Detection signals of various sensors that detect an engine operating state as well as detection signals of the aforementioned airflow meter 3 and the aforementioned air-fuel ratio sensor 12 are input to the electronic control unit 17. Besides, on the basis of the detection results of those sensors, the electronic control unit 17 controls the throttle valve 4, the injector 5, the VVT mechanism 7, the ignition plug 9, the air pump 15, the air switching valve 16, and the like, thereby performing controls for the engine.

Further, the electronic control unit 17 performs, as one of the controls for the engine, an air injection (AI) control of raising the temperature of a catalyst 13 through the supply of secondary air into exhaust gas, when the engine is started. In the AI control, secondary air is supplied into exhaust gas, and the fuel injection amount of the injector 5 is increased. The amount of unburned fuel components in exhaust gas is increased through the enrichment of the air-fuel ratio resulting from an increase in the fuel injection amount, and secondary air is supplied into exhaust gas, thereby burning the unburned fuel components in the exhaust passage 11 to raise the temperature of the catalyst 13.

More specifically, the AI control is performed in the following manner. That is, when a condition for performing the AI control is fulfilled, the electronic control unit 17 causes the air pump 15 to operate, and causes an AI counter to count up. The AI counter is a counter for measuring a time since the beginning of the operation of the air pump 15, and the value of the AI counter is counted up at intervals of a prescribed control cycle after the air pump 15 is operated. Then, when the value of the AI counter becomes equal to or larger than a prescribed AI increase criterial value, the electronic control unit 17 opens the air switching valve 16 to start supplying secondary air into exhaust gas, and starts increasing the fuel injection amount. The supply of secondary air and the increase in the fuel injection amount are continued until the temperature of the catalyst 13 rises to its activation temperature.

Furthermore, the electronic control unit 17 performs a variable valve timing control (a VVT control) for the intake valve by driving the VVT mechanism 7 during the operation of the engine. When the engine is cold, deterioration in driveability may be caused by the enrichment of the air-fuel ratio and the occurrence of hesitation due to the advancement of the valve timing of the intake valve. Therefore, the VVT control at the time when the engine is cold (a cold VVT control) is prohibited in principle, and the valve timing of the intake valve is held at its most retarded angle. However, in this embodiment of the invention, the cold VVT control is permitted only when a cold VVT control precondition described below is fulfilled.

It is determined whether or not the cold VVT control precondition is fulfilled, through the processing of a cold VVT control precondition fulfillment determination routine shown in FIG. 2. The processing of the routine is repeatedly performed at intervals of a prescribed control cycle by the electronic control unit 17 during the operation of the engine.

In the routine, it is determined that the cold VVT control precondition is fulfilled, when all conditions (A) to (G) described below are fulfilled (YES in all the determinations in S100 to S106) (S107). On the other hand, when one or more of the conditions (A) to (G) are unfulfilled (NO in one or more of the determinations in 5100 to S106), it is determined that the cold VVT control precondition is unfulfilled (S108).

The condition (A) is a condition that a condition for calculating a target displacement angle is fulfilled (S100: YES). The condition for calculating the target displacement angle is fulfilled when a condition that a rotational phase of an intake camshaft is set, a condition that an engine coolant temperature is normally detected, and a condition that a hydraulic pressure for driving the VVT mechanism 7 is not insufficient are all fulfilled, and also, the hydraulic pressure of the engine has risen or a VVT maximum retardation control is being performed after cold start.

The condition (B) is a condition that an accumulated intake air amount is equal to or smaller than a predetermined value (S101: YES). The condition (C) is a condition that there is no failure (abnormality) in the VVT mechanism 7 (S102: YES). The condition (D) is a condition that a deviation between a target rotational speed of the internal combustion engine and an actual rotational speed of the internal combustion engine is not very large, namely, that the deviation between the target rotational speed and the actual rotational speed is equal to or smaller than a predetermined value (S103: YES).

The condition (E) is a condition that the AI control is being performed (S104: YES). The condition (F) is a condition that a prescribed time has elapsed since the beginning of the AI control (S105: YES). More specifically, the condition (F) is a condition that the value of the aforementioned AI counter is equal to or larger than a prescribed cold VVT control performance criterial value (>an AI increase criterial value).

The condition (G) is a condition that the number of times of repeated start is equal to or smaller than a prescribed value (S106: YES). The number of times of repeated start is measured by a repeated start number-of-times counter. The value of the repeated start number-of-times counter is counted up when the engine is started. If an amount of fall in the engine coolant temperature from a previous trip is equal to or larger than a predetermined value when an ignition switch is on, the value of the repeated start number-of-times counter is cleared. It should be noted that the aforementioned prescribed value is set to “2” in this embodiment of the invention, and that the cold VVT control precondition is unfulfilled when the number of times of repeated start becomes equal to or larger than 3.

Subsequently, the advantageous effects of this embodiment of the invention configured as described above will be described. As shown in FIG. 3, when a condition for performing the AI control is fulfilled at a time t0, the air pump 15 operates, and the counting-up of the AI counter is started. Then, when the value of the AI counter becomes equal to or larger than the AI increase criterial value at a time t1, the air switching valve 16 is opened to start supplying secondary air into exhaust gas, and the fuel injection amount is increased (the AI increase is carried out).

Afterwards, when the value of the AI counter becomes equal to or larger than the cold VVT control performance criterial value at a time 2, the cold VVT control performance condition is fulfilled, and the cold VVT control is started. When the cold VVT control is started, the target VVT advancement amount is increased from “0”, and the valve timing of the intake valve is advanced.

As shown in FIG. 4, when the AI control is started, the AI increase is carried out, that is, the fuel injection amount is increased, and therefore, the air-fuel ratio becomes rich. In this embodiment of the invention, the cold VVT control is permitted after the air-fuel ratio is enriched due to this AI increase.

It should be noted that the cold VVT control is started upon the lapse of the prescribed time after the beginning of the AI control in this embodiment of the invention. Thus, there is a time lag between a time when the air-fuel ratio starts to be enriched through the increase in the fuel injection amount in the AI control, and a time when the air-fuel ratio starts to be enriched through the performance of the cold VVT control. This avoids the situation where torque is abruptly increased as a result of the enrichment of the air-fuel ratio by the cold VVT control starting simultaneously with the start of the enrichment of the air-fuel ratio by the AI control.

Further, in this embodiment of the invention, as shown in FIG. 5, when the value of the repeated start number-of-times counter is equal to or smaller than 2, the cold VVT control precondition can be fulfilled. The value of the repeated start number-of-times counter is counted up every time a start flag indicating the completion of the start of the engine is turned on. The cold VVT control is permitted when other conditions are fulfilled. On the other hand, when the value of the repeated start number-of-times counter becomes equal to or larger than 3, the cold VVT control precondition is unfulfilled, and the cold VVT control is prohibited.

It should be noted that the value of the repeated start number-of-times counter is cleared if the time from the stoppage of the engine to the restart of the engine is sufficiently long and the engine coolant temperature has fallen by a predetermined value or more since the end of a previous trip when the ignition switch is turned on. Thus, after the value of the repeated start number-of-times counter is cleared, the cold VVT control is permitted again.

As described above, in this embodiment of the invention, the cold VVT control is limited in accordance with the number of times of repeated start, for the following reason. When the engine is started, a large amount of fuel adheres to a wall surface of an intake port. Thus, when the internal combustion engine is repeatedly started before the lapse of an engine stop time that is sufficient to evaporate the fuel that has adhered to the intake port, the amount of the fuel that has adhered to the intake port increases every time the engine is started. Then, if the cold VVT control is performed with an excessive amount of fuel adhering to the intake port when the engine is cold, the air-fuel ratio is greatly enriched, and a combustion failure may be caused. Thus, in this embodiment of the invention, when the number of times of repeated start becomes large, the cold VVT control is prohibited.

The control apparatus for the internal combustion engine according to this embodiment of the invention described above can achieve the following advantageous effects. (1) In this embodiment of the invention, the condition for exceptionally performing the VVT control, which is prohibited in principle when the engine is cold, includes the condition that the control of increasing the fuel injection amount through the AI control is being performed. If the cold VVT control is performed when the AI control is not being performed, driveability is deteriorated by the enrichment of the air-fuel ratio or the occurrence of hesitation. However, when the AI control is being performed, the air-fuel ratio has been enriched due to the AI increase, i.e., the increase in the fuel injection amount, and deterioration in driveability has already been caused. Thus, during the performance of the AI control, the cold VVT control can be performed without the need of worrying that deterioration in driveability is caused. Further, since the fuel injection amount has been increased through the AI control, hesitation due to deficiency in burned fuel is avoided during acceleration. Thus, according to this embodiment of the invention, the variable valve timing control can be performed when the engine is cold, without increasing the range of deterioration in driveability.

(2) In this embodiment of the invention, the condition for performing the variable valve timing control when the engine is cold includes the condition that the prescribed time has elapsed since the beginning of the control of increasing the fuel injection amount. Thus, a time lag occurs between the beginning of the control of increasing the fuel injection amount and the beginning of the variable valve timing control. Thus, an abrupt increase in torque can be suppressed.

(3) In this embodiment of the invention, the condition for performing the variable valve timing control when the engine is cold includes the condition that the number of times of repeated start of the internal combustion engine is equal to or smaller than the prescribed value. Thus, in a state where an excessive amount of fuel has adhered to the intake port as a result of repeated start, the cold VVT control is prohibited. This avoids the situation where the air-fuel ratio is abruptly enriched through the performance of the cold VVT control.

It should be noted that the foregoing embodiment of the invention can also be implemented after being modified as follows. In the foregoing embodiment of the invention, the number of times of repeated start for prohibiting the cold VVT control is equal to or larger than 3. However, the number of times of repeated start for prohibiting the cold VVT control may be appropriately changed in accordance with the state of adhesion of fuel to the wall surface of the intake port of the internal combustion engine.

In the foregoing embodiment of the invention, when the number of times of repeated start of the internal combustion engine becomes large, the cold VVT control is prohibited. However, in the case where the amount of the fuel that has adhered to the wall surface of the intake port is not greatly increased in response to repeated start, the cold VVT control may not be limited in accordance with the number of times of repeated start.

In the foregoing embodiment of the invention, the cold VVT control is permitted after the prescribed time has elapsed since the beginning of the increase in the fuel injection amount through the AI control. However, in the case where driveability is not significantly deteriorated even when the enrichment of the air-fuel ratio resulting from the cold VVT control starts simultaneously with the start of the enrichment of the air-fuel ratio resulting from the increase in the fuel injection amount, the cold VVT control may be started simultaneously with the increase in the fuel injection amount through the AI control.

In the foregoing embodiment of the invention, the cold VVT control is permitted on the condition that the fuel injection amount is increased through the AI control for raising the temperature of the catalyst. However, the cold VVT control may also be permitted when the fuel injection amount is increased in a control other than the AI control. For example, the fuel injection amount may be increased in accordance with a shift change in a transmission. If a condition that the fuel injection amount is increased in accordance with the shift change in the transmission is set as the condition for performing the cold VVT control, the variable valve timing control can be performed when the engine is cold, without causing deterioration in driveability.

Next, on the basis of a technical concept that can be grasped from the foregoing embodiment of the invention and its modification examples, in the control apparatus for the internal combustion engine according to the invention, the valve timing of the intake valve may be most retarded while the variable valve timing control is prohibited when the engine is cold.

In the case where the valve timing of the intake valve is most retarded while the variable valve timing control is prohibited, the valve timing of the intake valve is advanced when the variable valve timing control is performed. Then, when the valve timing of the intake valve is advanced, the valve overlap of the intake valve and the exhaust valve increases, the blowback of combustion gas to the intake port increases, and the gasification of the fuel that has adhered to the wall surface of the port is promoted to enrich the air-fuel ratio. Thus, in the case where the valve timing of the intake valve is most retarded while the variable control is prohibited, there is a possibility that driveability may be deteriorated due to the performance of the variable valve timing control of the intake valve when the engine is cold, and it is desirable to prohibit the performance of the variable valve timing control in principle. If the invention is applied to this configuration, the variable valve timing control of the intake valve can be permitted under a certain condition when the engine is cold, without increasing the range of deterioration in driveability. 

1. A control apparatus for an internal combustion engine, comprising: a controller that performs a variable valve timing control to change a valve timing of an intake valve, wherein a condition for performing the variable valve timing control when the engine is cold includes a condition that a control of increasing a fuel injection amount is being performed.
 2. The control apparatus according to claim 1, wherein the control of increasing the fuel injection amount is performed to raise a temperature of a catalyst.
 3. The control apparatus according to claim 1, wherein the condition for performing the variable valve timing control when the engine is cold includes a condition that a prescribed time has elapsed since beginning of the control of increasing the fuel injection amount.
 4. The control apparatus according to claim 1, wherein the condition for performing the variable valve timing control when the engine is cold includes a condition that a number of times of repeated start of the internal combustion engine is equal to or smaller than a prescribed value.
 5. The control apparatus according to claim 4, further comprising a repeated start number-of-times counter that counts a number of times of repeated start of the internal combustion engine, wherein a value of the repeated start number-of-times counter is cleared when an engine coolant temperature has fallen by a predetermined value or more since end of a previous trip.
 6. A control method for an internal combustion engine, comprising: determining whether or not a condition for performing a variable valve timing control when the engine is cold is fulfilled; and performing the variable valve timing control if it is determined that the condition for performing the variable valve timing control when the engine is cold is fulfilled, wherein the condition for performing the variable valve timing control when the engine is cold includes a condition that a control of increasing a fuel injection amount is being performed.
 7. The control method according to claim 6, wherein the control of increasing the fuel injection amount is performed to raise a temperature of a catalyst.
 8. The control method according to claim 6, wherein the condition for performing the variable valve timing control when the engine is cold includes a condition that a prescribed time has elapsed since beginning of the control of increasing the fuel injection amount.
 9. The control method according to claim 6, wherein the condition for performing the variable valve timing control when the engine is cold includes a condition that a number of times of repeated start of the internal combustion engine is equal to or smaller than a prescribed value. 